The potential of fluoride glass fibers for ultra low loss applications, particularly in long wavelength, i.e. infra-red systems, has been recognized for some time. Potential applications include low loss optical communication systems in which repeaters can be essentially eliminated for many long distance links. The theoretical loss of these materials due to intrinsic Rayleigh scattering and the multiphonon absorption edge loss is 0.001 dB/km at 2.5 .mu.m and should allow a repeaterless communications link of several thousand kilometers. Other photonic device applications include optical fiber amplifiers, filters, upconversion devices, multiplexers, etc. Other industrial applications include diagnostic and surgical tools in medicine, power transmission, high temperature sensing equipment, high power lasers, all of which may require low loss, long wavelength light transmission for which these fibers are especially suited.
The commercial potential of fluoride fibers has not been realized to date due to their poor chemical durability and low mechanical strength. Defects on drawn fiber surfaces can occur from chemical imperfections and impurities. Moreover, fluoride glasses are inherently soft, and slight mechanical imperfections from drawing and coating are largely unavoidable. While small surface defects in silica glass fibers occur routinely, these are essentially benign when coated with conventional polymer materials. By contrast, small surface defects in fluoride glass fibers grow in spite of conventional polymer coatings under most ambient conditions, especially under humid conditions. Moreover, the heavy metal fluoride glasses, i.e. zirconium fluoride glasses, have poor chemical stability especially in the presence of water, forming zirconium hydroxide crystallites on exposed surfaces. These crystallite formations on the surface produce crack initiation sites on the glass surface and drastically reduce fiber strength, typically from 250 ksi to a few tens of ksi.
The extreme moisture sensitivity of these fibers has been addressed by many workers using a variety of hermetic coating materials which so far have proved relatively ineffective in producing high strength fibers.
Coatings applied to the as drawn fiber present difficult materials and process engineering problems, although a wide variety of such techniques have been attempted. See e.g., G. A. Al-Jumaily et al, Mater. Sci. Forum Vol. 6, p. 721, (1985); P. C. Schultz et al , Mater. Sci. Forum Vols. 19-20, pp. 419-430, (1987).
More promising, are bulk glass coatings applied to the fluoride glass preform prior to drawing. This allows the use of more or less conventional fiber manufacture techniques and equipment. These techniques have been moderately successful, producing losses of the order of 0.5 dB/km, which makes many short length fiber device applications feasible. In particular, these glasses are now recognized as ideal host materials for many near-IR active fiber device applications because of their long-wavelength multiphonon absorption edges.
In most cases the heavy metal fluoride glasses are so-called ZBLAN glass or modifications thereof. ZBLAN is an acronym for a mixed glass of fluorides of zirconium, barium, lanthanum, aluminum and sodium and has been the material of choice in many of the investigations in the development of fluoride optical fibers. ZHBLAN is an acronym for a mixed glass of fluorides that has hafnium in addiction to the components of the ZBLAN glasses.
Schultz et al used ZBLAN glasses and experimented with various inorganic oxide, nitride, and carbide protective coatings using RF-sputter coating. They identified MgO as a good candidate for an effective moisture barrier, but reported no results on actual fibers. See P. C. Schultz, L. J. B. Vacha, C. T. Moynihan, B. B. Harbison, K. Cadien, R. Mossadegh, "Hermetic coatings for bulk fluoride glasses and fibers", Materials Science Forum, Vol. 19-20 19-20, pp. 343-352 (1987). Buhler et al have reported good bulk glass protection on ZBLA glass (ZBLAN glass without sodium fluoride) using e-beam evaporated SiO.sub.2 and Ta.sub.2 O.sub.5 films. See M. Buhler, J. Edinger, H. K. Pulker, M. Reinhold, B. Bendow, O. El-Bayoumi, "Optical and protective coatings for heavy-metal fluoride glasses prepared by reactive ion plating", Materials Science Forum, Vols. 19-20, pp. 353-362 (1987). However, a large thermal expansion coefficient differences exists between these coating materials and ZBLAN, which prohibits their use as bulk coating materials on optical fiber preforms. Vacha et al have developed a phosphate glass overcladding for preforms that has a glass transition temperature of 247.degree. C. and a thermal expansion coefficient of 17.4.times..sup.-6 /.degree. C. They used the rotational casting method to make phosphate tubes, and sequentially poured the molten glasses for the clad and core into a rotating cylindrical mold to produce the multimode optical fiber preform. The use of this overclad enhanced the fracture strength significantly. See L. J. B. Vacha, P. C. Schultz, C. T. Moynihan, S. N. Crichton, "Flox fibers: fluoride glasses with oxide overclad, Materials Science Forum, Vols. 19-20, pp. 419-430 (1987). Orcel et al have studied various metal oxides in phosphate glasses and developed a chemically durable phosphate glass suitable for fluoride glass protection. See G. Orcel, D. Biswas, M. R. Shahriari, T. Iqbal, G. H. Sigel, "Development of a new glass for fluoride fiber overclad", Materials Science Forum, Vol. 67, pp. 569-574 (1991). Phosphate glasses in general have been studied in great detail. See e.g. N. H. Ray, C. J. Lewis, J. N. C. Laycock, W. D. Robinson, "Oxide glasses of very low softening point. Part 1,2 Preparation and properties of some lead phosphate glasses", Glass Technology, Vol. 14, pp. 50-59 (1973); B. C. Sales, L. A. Boatner, "Optical, structural, and chemical characteristics of lead-indium phosphate and lead-scandium phosphate glasses", J. Amer. Ceram. Soc., Vol. 70, pp. 615-621 (1987); Y. B. Peng, D. E. Day, "High thermal expansion phosphate glasses", Glass Technology, Vol. 32, p. 166 (1991). These glasses have low softening and melting temperatures, good temperature durability, and good mechanical properties. They also have a large coefficient of thermal expansion, and were originally developed for sealing metal leads. The temperature dependence of their viscosity is similar to silicate glasses and not nearly as steep as the fluorozirconate glasses. Glass materials in this category have base compositions of 50-70 mol % P.sub.2 O.sub.5, 10-30 mol % PbO, and 10-20 mol % of oxides of Li,K, and Na. Alkaline earth oxides, MgO, BaO and CaO, and metal oxides like CdO and V.sub.2 O.sub.5 are added to improve durability and decrease the water solubility.
In a recent study, Hartmann et al have used a phosphate glass with a 227.degree. C. glass transition to overclad ZBLAN optical fiber preforms. See M. Hartmann, G. H. Frischat, K. Hogerl, G. F. West, "Resistant oxide coatings for heavy metal fluoride glasses", Journal of Non-Crystalline Solids, Vol. 184, pp. 209-212 (1995). They coated the preforms using a dip coating process using a melt at 500.degree. C. and observed crystallization at the fluoride glass-phosphate glass interface. On decreasing the melt temperature to 350.degree.-370.degree. C. they obtained a good transparent coating. The phosphate glasses they used have high melting temperatures and therefore do not allow lower temperatures for dip-coating. The work of Hartmann et al focuses on the temperature regime above the crossover point in the viscosity vs. temperature plot of the fluoride and phosphate glasses (see FIG. 3 of Hartmann et al). At the dip-coating temperatures used by Hartmann et al the viscosity of the fluoride glass being coated is very low and significant crystallization results.